Flexible electrical insulating layer



7 Oct. 9, 1945. P. ROBINSON 2,386,634

FLEXIBLE ELECTRICAL INSULATING LAYER Filed March 20, 1940 FEECENT ENAMLlN COflT/A/G o a 4 e a /0 /a 14- la 20 22 a4 RAT/0, 5y WEIGHT, OF ENAMEL1-0 B/IVPEZ /N COAT/N6 su pg vs o v PRESTON ROBINSON INVENTOR.

W MW

ATTORNEY.

Patented Oct. 9, 1945 FLEXIBLE ELECTRICAL INSULATING LAYER PrestonRobinson, William'stown, Mass., assignor toSprague Electric Company,North Adams, Mass, a corporation of Massachusetts Application March 20,1940, Serial No. 325,043

' 6 Claims.

same. An-important application of the invention is for theheat-resistant insulating coating of electrical conductors and theinvention will be described in'this connection.

The present invention relates to electrical insulating layers and to aprocess of making the a above requirements.

One object of the invention is to provide novel '7 insulatin layerswhich withstand high temperatures and exhibit also at such temperatureshigh insulating resistivity and dielectric strength.

Another object of the invention is to provide an insulatinglayer whichcomprises a high proportion of an inorganic insulating refractorymaterial and which layer is inherently flexible, and of great mechanicalstrength.

A further object of the invention is 'to provide layers possessing theabove desirable characteristics irrespective of whether they areappliedas a coating to a base or are used as independent layers.

A still further object is to provide a refractory insulating coatingwhich is well adherent to a base and resistant to mechanical stresses.

Another object of the invention is to provide an insulating coatin whichis adapted to be fused to a base and which in fusion does not undergo ashrinkage to an extent deleterious to the properties of the coating.

These and further objects of the invention will appear as thespecification progresses.

As a result of an extended study of the factors influencing the physicaland electrical characteristics of insulating layers, I have found thatto obtain a satisfactory insulating layer of which a refractory materialis the main constituent and which exhibits a high degree of mechanicalstrength and flexibility, certain requirements relative to theconstituents and the method of depositio'n thereof must be met.

I have found, for example, that to obtain a coating having theabove-described desired characteristics, the refractory material must beheld together andbonded to the base to which it is applied, by a binderwhich exhibits, or can be made to exhibit, a high degree of surfaceadhesion. Not to impair the electrical characteristics of the refractorymaterial, the binder must also possess low electrical losses, highinsulating resistivity and high resistance to humidity.

I have iurtherfore found that the group of compounds which exhibitelastic and rubber-like properties, for example, rubber latex, neoprenelatex, the butadiene rubbers, polymerized isoprene, derivatives ofisoprene, and resins of the The above compounds all essentially consistof .high molecular weight organic polymers with elastic properties andhave been jointly classified as Elastomers by H. L. Fisher (see articlein Industrial and EngineeringChemistry, volume 31, No. 8, pages 941 to945 incl.)

I have furtherfore found that to enable these elastomers to exhibit therequired high adhesion forces, they have to be in a finely divided form,and that to realize their full advantage as binders, the elastomer andthe frit of the inorganic insulating material must be mixed while bothare in a finely divided form. This can be achieved, for example, bysuspending the binder and the frit in a suitable suspension medium fromwhich the coating is then electrophoretically deposited on the conductorto be coated.

Why the above requirements have to be met and the exact manner in whichthe novel results are brought about are not fully known to me. I believehowever, that by finely dividing the clastomer and the frit and bysimultaneously electmphoretically depositing same, the individualelastomer particles form in the deposited coating strong links, whichinterconnect the adjacent frit particles and bond them to each other andto the conductor. I have found that in accordance with the inventionexcellent coatings are obtained which comprise or even more by weight offrit and thus 10% or less of the binder.

While the best results to practice my invention can be obtained by usingelastomers as binders, I have found that for certain purposes fairlysatisfactory results can also be obtained by using the process of theinvention with binders formed of substances which are not trueelastomers but can be made to exhibit elastomerlike properties. I havefound that certain resins through addition of a high proportion of aplasticizer can be made to exhibit such elastomer-like behavior as abinder. This is for instance the case with polystyrene by adding to itmore than 25% (by weight) dibutyl phthalate, polyamylnaphthalene, orsimilar plasticizing agents.

As has been stated, my invention is of general usefulness to provideinsulating layers of inorganic materials which maintain high dielectricstrength and insulating resistivity at elevated temperatures. Suchlayers whether s d in. themselves or applied to a base, show greatflexibility and good mechanical strength.

If my invention is used for dielectric layers or insulating coatingsapplied to a base, the high My invention has the further advantage thatC when fused coatings are desired, fusion of the coating does not causesuch shrinkage of the coating as to deleteriously afiect its properties.Thus, fused coatings made according to my invention are free ofimperfections, wealr spots and other shortcomings.

. While it has been heretofore proposed to pro vide insulating layersconsisting of a frit and a binder in a small proportion, such priorlayers have not been satisfactory due to the inherent shortcomings ofthe processes used in their manufacture. form such layers by extruding amixture consisting of the frit and binder and of a solvent for thebinder, the solvent being required to "Wet the surfaces-of the iritparticles. Layers so formed cannot, however, be made in thicknesses lessthan about .02". Furthermore, the layers For example, it has beenproposed to 'assacaa oxides of aluminum, magnesium and titanium; or ofother refractory insulating materials, such as talc, silica, or mica.

As binder, an elastomer is used which is also provided in a finelydivided form, and which may be, for example, rubber latex, neoprenelatex, butadiene rubber, polymerized isoprene, derivatives of isoprene,and/or resins oi the acrylic ester type. i

As the suspension medium I usually use water and preferably distilledwater, although other obtained by this process are non-uniform inthickness and exhibit numerous weak spots and imperfections.

Another method of obtaining insulating layers of the above type,consists in applying to the sursuitable binder. Such a method does notproduce a coating having a high proportion of the refractory material,because such fibrous refractory materials contain a largeproportion andmore) of voids. Furthermore, the layers so obtained are brittle and canonly be produced in comparatively large thicknesses.

Unlike the above described layers of the prior art, the layers or theinvention possess excellent mechanical strength and flexibility not onlyin thicknesses of the order of .005", but down to thicknesses of .00025and less, and even such thin;

layers can be obtained with comparative ease.

My invention-finds its chief use for layers less than .005" thick and isparticularly directed to the'obtainment of such layers.

My invention will be further described with reference to the appendeddrawing in which:

' Figure 1 is a schematic illustration of a suitable apparatus forcarrying out the process of my invention. x I, i

Fig. 2 is an enlarged cross-sectional view of a wire element providedwith an insulating coating in accordance with the invention;

Fig. 3 is a graph giving the relation between the percentage of the fritin the coating, to the ratio of frit to elastomer of the coatingsolution. Referring to Fig. 1, a wire I5 is passed for its coatingthrough a coating cell comprising in a container ill, a coating solutionll consisting of suspended particles of a suitable frit and of anelastomer in a. suitable suspension medium.

Th 1 frit is preferably in particles having a size of 1 to 2 microns andmay consist of one or of a mixture of heat-resistant, inorganicinsulating materials of a vitreous type, such as porcelain "enamel, leadborate or other enamels or borate glasses; or of refractory metaloxides, such as the poorly conducting media in which neither the fritnor the binder is strongly soluble, can be used.

As a rule, I first prepare or obtain a suspension of the elastomer, andthen add to it the frit in the desired proportion by weight.

While any of the elastomers above mentioned.

are suitable for my invention, I have found that the resins of theacrylic ester type excel in this of finely divided particles in anamount later more fully discussed, so that this minture forms thecoating solution ii. i

As a rule, the particles of the frit and elastomer carry negativecharges, and the conductor is is positive by the addition to thesolution oi suitable polyvalent cations.

The percentage which the insulating frit ins-- teriai constitutes of theresulting coating, is determined by the ratio of the frit and the binderI maintained in the coating solution during the electrophoreticdeposition.

This relationship I have shown in Fig. .3 for a coating deposited from acoating solution consisting of an aqueous suspension of porcelain enamelfrit and of the elastomer Appretan A. For other frits and elastomersimilar relationships exist.

In Fig. 3 the abscissae represent the ratio by weight of the frit andthe elastomer in the coating solution, and the ordinates the percentageof frit-in the deposited coating. As appears from the graph, a coatingcomprising more than 80%- frit is obtained with a coating solution inwhich the ratio of frit to binder is greater than 5 to 1.

- Ifthis ratio is about 20 to 1, a coating, in which the frit content isincreased to about (by weight), is obtained. Even a coating with such ahigh proportion of frit, possesses excellent meunwound from a spool I6mounted on a rotatable shaft l1, and passes on its way to the coatingcell around a mandrel I8 and over a. guide pulley l9.v

The wire 'l 5preferably passes through the coating cell as a loop,reversing at the bottom of the cell, and for this purpose there isprovided a pulley 3| supported from the bottom of the container by abracket 30. On its outgoing end the wire l5 In the oven the moisture isremoved from th coating in various ways, for example, by passing throughthe oven a heated air blast from a jet 22 connected to a suitable source(not shown). From the oven H the wire passes to and is wound upon aspool 23.

In case such coatedwire is to be used for the winding oi coils and thecoating is to be fused to the wire, such fusing takes place after thecoil has been wound, i. e. after the wire has been bent to its finalshape. The temperature required to fuse the coating is determined by thefusion point of the specific refractory frit used. The electric currentfor the coating is supplied by a suitable source of direct current,indicated as a battery 24, the negative pole of which is connected tothe container I0, and the positive pole of which is connected through anammeter 25 to a contact brush 29 which contacts with the mandrel l8.Through the mandrel 18 the wire I is thus connected to the positive poleof the source 24. the source 2!.

Suitable driving means A voltmeter 25 is connected across (not shown) Yrotate spools I6 and 23 and cause the wire l5 to unwind from-spool I6,pass through the coating cell and the oven, and to wind upon spool 23;

Figure 2 shows an enlarged cross section of an electrical conductorprovided with a coating of my invention. The coating comprises fritparticles 21 intimately bonded to each other and to the wire l5 by anelastomer binder 28.

' I will further illustrate my invention by means of the followingspecific examples:

For the coating of a copper wire .05" in diapproximately 95% frit andonly 5% of binder. Even with such a high proportion of frit, the coatingobtained exhibits a high degree of flexibility and mechanical strength.The soecoated wire can be bent into a helix about a mandrel 0.4" indiameter, or eight times the diameter of the wire, without any damage tothe coating. A coating comprising the above frit fuses at ap-.proximately 725 C.

It should be noted that my invention because of the high degree offlexibility, the high mechanical strength and the low shrinkage infusion ameter, I prepare a coating solution of 200 cc.

of "Appretan .Aconsisting of an aqueous suspension of 52 grams ofacrylic ester resin--to which I add a frit consisting of 220 grams ofporcelain enamel and 94 grams of silica. Through this coating solutionthe wire is passed at a speed of one foot per second, whereby a lengthof two feet of the wire is immersed at a time. By applying a coatingvoltage of 5 volts across the coating cell, a current of about 50milliamperes per inch of immersed length of wire passes through the celland causes a coating of the thickness of .002" to deposit on the wire.

The voltage as well as the time of deposition may be varied, whereby ingeneral the thickness of the deposited coating, is roughly proportionalto the voltage used, and thus the time of deposition can be reduced bythe use of a correspondingly higher voltage.

To dry the wire I use an air blast heated to a temperature of 100 C. f 1

The coating so obtained" consists of approximately 85% (by weight) fritand 15% of binder. A wire so coated can be tied into a knot without 4any damage to the coating.

.does not cause such shrinkage of the coating as to deleteriously affectits properties. By using the same type of wire as above and under thesame conditions of coating, but using a coating solution of 200 cc. ofwater and 100 cc.

of "Appretan A.containing 26 grams of acrylic ester resinto which isadded 250 grams of porcelain enamel, I obtain a coating which containsof the coating, has for the first time made it possible to provide coilsthe insulation of which consists of an integrally fused coating of arefractory material. Coils of even the most complicated shapes can be soprovided while the wire is flexible, and in the subsequent fusion of thecoating an inflexible and highly stable assembly is formed.

Furthermore, while the coating of the invention is particularly adaptedfor fused integral coatings, coatings of the invention exhibit highlydesirable electrical properties even if not fused. Such coatings areheat resistant, moisture-resistant, have low dielectrical lossesand highinsulating resistance. A coating formed in accordance with the firstabove example but without being fused, has a breakdown voltage in excessof 1000 volts per mil of thickness and a resistivity of approximately1000 megohms.

As above stated, the layers of the invention are also applicable as adielectric for electrical condensers either as independent layers or asan integral coating on the condenser electrodes.

,An excellent dielectric layer can be provided as an integral coating onthe surface of a condenser electrode by electrophoretic deposition fromacoating solution of 300 cc. of water, 200 cc. of CorialBottomcontaining grams of resin and 400 grams of titanium dioxide. Theelectrode to be coated is made the anode in the coating cell and byapplying to the cell a coating voltage of 30 volts for one second, alayer approximately .00025 thick is deposited on the electrode. A layer.001" is obtained when a coating voltage of 60 volts applied for twoseconds is used.

A layer deposited from the above coating solution contains approximately(by weight) titanium dioxide and 20% binder. Such a. layer has adielectric constant of approximately 20, a power factor less than 0.5%,a resistivity greater than 100,000 megohms and a voltage breakdownstrength in excess of 1000 volts per mil thickness.

Other frits can also be used for obtaining good dielectric layers, forexample, a layer consisting of talc as a, frit and 15% Corial Bottom asthe'elastomer binder, has a resistivity in excess of frequencies of lessthan 0.25%.

While I have described my invention by means of speciflc examples and ina specific application, I do not wish to be limited thereto as variousmodifications will occur to those skilled in the art without departingfrom the spirit and scope of the invention.

What I claim is:

1. The method of applying to a flexible electrical conductor aninsulating layer comprising the steps, forming in a susepnsion medium asuspension of afinely divided, refractory dielectric material and afinely divided acrylic ester resin, said refractory material and resinbeing in said suspension in a ratio greater than 5 to 1 respectively andless than about 20 to 1 respectively, and electrophoretically depositingfrom said suselastomer, said refractory material and elastomer being insaid suspension in a ratio greater than 5 to 1 respectively and lessthan 20 to 1 respectively, and electrophoretically depositing from saidsus pension a layer on said flexible electrical com ductor whichcomprises in excess of 80% by weight of said refractory material.

aeeaeee 3. The method which comprises fusing the in-v sulating layer ofa coil of the insulated electrical conductor referred to in claim 1.

d. The method which comprises fusing the insulating layer of a coil ofthe insulated electrical conductor referred to ,in claim 2.

5. A highly flexible, insulated, electrical conductor produced inaccordance with the process defined in claim 1.

6.-A highly flexible, insulated, electrical conductor produced inaccordance with the process defined in claim 2. 1

PRESTON ROBINSON.

